June 14, 2013

Menopause is caused by male preference for younger women (or is it?)

My own half-baked idea is that because humans have longer childhoods and are very dependent on their mothers during their childhoods, there's an advantage to live past their reproductive years.

If a couple of women cease reproduction at 40 but one of them dies at 40 and the other at 50, there's a greater probability that the latter's offspring will survive and reproduce themselves.

So, increasing the gap between menopause and death increases the chances of success for offspring (because their mother lives longer to support them into adulthood). As childbirth becomes ever more dangerous (because of normal aging process) it "pays more" to stay alive and raise the kids already born than to gain more kids, at the risk of losing the ones you already have.

PLoS Comput Biol 9(6): e1003092. doi:10.1371/journal.pcbi.1003092

Mate Choice and the Origin of Menopause

Richard A. Morton et al.

Human menopause is an unsolved evolutionary puzzle, and relationships among the factors that produced it remain understood poorly. Classic theory, involving a one-sex (female) model of human demography, suggests that genes imparting deleterious effects on post-reproductive survival will accumulate. Thus, a ‘death barrier’ should emerge beyond the maximum age for female reproduction. Under this scenario, few women would experience menopause (decreased fertility with continued survival) because few would survive much longer than they reproduced. However, no death barrier is observed in human populations. Subsequent theoretical research has shown that two-sex models, including male fertility at older ages, avoid the death barrier. Here we use a stochastic, two-sex computational model implemented by computer simulation to show how male mating preference for younger females could lead to the accumulation of mutations deleterious to female fertility and thus produce a menopausal period. Our model requires neither the initial assumption of a decline in older female fertility nor the effects of inclusive fitness through which older, non-reproducing women assist in the reproductive efforts of younger women. Our model helps to explain why such effects, observed in many societies, may be insufficient factors in elucidating the origin of menopause.

On one hand, the "just so" stories that evolutionary biology encourages researchers to consider are the right kind of answers. But, on the other hand, these stories are often not unique solutions, so coming up with one does not mean that you've solved the problem. Indeed, often, as in this case, where there is not one decisive and obvious story involved, you there will be many possible explanations none of which is decisively a strong candidate for being the right answer. It takes more evidence than comparisons of story telling quality to know which possible story is right (or even if the only story anyone has come up with is right). In the case of menopause, I think that you'd really have to know more about the biochemical processes and genes that are activated in the process to even come close to getting a good answer.

I was thinking that nature seems to be particularly harsh to women in childbirth. No matter how painful and laborious, all that matters from an evolutionary perspective is that the mother and child suvive and recover, although they often do not. Women dying in childbirth seems to be nature's way of keeping childbirth tenable as the next generation will be better adapted. Childbirth seems to be a case where human adaptation hovers around the tenable threshold.

Perhaps labour has an evolutionary role to prepare women for the pain and sacrifices that await them when they raise the kids. :)

Coincidentally I'd been reading around in regards to menopause. In western countries where people have access to good nutrition, menopause occurs at a later age than in countries where malnutrition/undernutrition is prevalent.

For example in India the average at for menopause is 44 years. This has been attributed to nutritional deficiency. It only makes sense that women are breeding at younger ages. Given that women in countries where age of menopause is on average 52 years, their fertility declines precipitously after age 35. So, deducting from this, the average woman in India has reduced fertility before age 30.

So possibly the penchant for men to breed with younger women has taken this into consideration and the theory posited in the news today is the opposite to what is actually happening. It's not genetics, it's nutrition and the accumulation of disease load.

What about super fertile women with ashkenazi jew ancestry who can reproduce up to 60 years old?

It seems like the recombination/mutation rate would be more involved here than anything else. Higher recombination rate means you would get too many birth defects after a certain point. Certainly conscious selection has to be very unlikely.

Rather stupid article. So, men prefer younger women. Sure. Does that mean slightly older women suffer from a lack of sex? Just silly. Women of almost any age have sex. Male preference for younger women means nothing if the older women still have sex. Or, do they mean that older women can't form supportive attachments to men, because the guys are all off chasing younger women? Show some evidence, and not another just-so story.

Since the percentage of miscarriages and severe birth defects increases tremendously after age ~30-32 for women, menopause is a somewhat cleaner way for nature to achieve longer age and it's societal benefits without the associated detriment.

Elephants and whales have menopause so i think the correct explanation will work for them also. I'm not sure if this does or not.

.Personally I think the grandmother hypothesis makes the most sense. Staying alive after menopause helped the grand-children. You see this all the time - at least in more bluecollar or rural populations that live close together - with female relatives forming their own private childcare network.

I think the problem with menopause was explained well before, why the need of new theories.Women need to be alive long enough to raise their last child to maturity. It is not that important for father to be around at all times, so this does not concern men.Humans have one of the highest periods of juveniles reaching maturity of all mammals - around 13-15 years, hence the need of menopause.No wonder elephants and whales, who reach maturity at 10-15 need this, too. Most other mammals can breed at 1 or at most 2 years of age, so mothers can afford to give birth to the end.

Grey,"Elephants and whales have menopause so i think the correct explanation will work for them also. I'm not sure if this does or not.Personally I think the grandmother hypothesis makes the most sense."

It would make sense for elephants since grandmothers run elephant herds.

"Menopause also has been reported in elephants,[123] short-finned pilot whales[124] and other cetaceans,[125] as well as in a variety of other vertebrate species including the guppy,[126] the platyfish, the budgerigar, the laboratory rat and mouse, and the opossum, as well as some whales.[127] However, with the exception of the short-finned pilot whale, such examples tend to be from captive individuals, and thus they are not necessarily representative of what happens in natural populations in the wild."

So only observed in the wild in pilot whales. I'd read elsewhere there were a few whale species including killer whales but i guess that may just have been in captivity.

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